Carbon dioxide emissions to the atmosphere have risen steadily since the beginning of the industrial revolution. At present, worldwide combustion of fossil fuels emits about 22 Gt of carbon dioxide to the atmosphere annually. The measured annual increase in atmospheric carbon dioxide is approximately 13 Gt. The difference between total output, which includes some additional emissions from deforestation and other anthropogenic sources, and the observed increase in atmospheric carbon dioxide is absorbed into natural sinks like the ocean and the biosphere. The substantial absorption indicates that the current state of the atmosphere is far from a steady-state equilibrium. The level of atmospheric carbon dioxide has risen by 30 percent from its pre-industrial value of 280 ppm to about 365 ppm today. Most of this rise (about 60 ppm) has occurred during the past 50 years.
The size of readily accessible fossil fuel deposits is extremely large. Easily accessible oil and gas may be limited, but oil shales, tar sands and coal deposits are virtually inexhaustable. Coal deposits alone are estimated at 10,000 Gt, which may be compared to a worldwide annual consumption of about 6 Gt of carbon. Methane hydrate deposits have become of interest recently, and may dwarf all other carbon resources. It can thus be concluded that fossil fuel resources are not ultimately limited by availability, or even for that matter by the cost of extraction. Past history suggests that technological advances can be expected to keep up with a gradual degradation of the quality of the energy resources. Furthermore, various hydrocarbon sources can be regarded as virtually interchangeable at some incremental cost over current energy costs.
Today, fossil energy contributes about 85 percent of the world energy supply. It is the cheapest, most readily available energy source. Thus, fossil energy is likely to remain the dominant energy resource for satisfying the growing world energy demand. World energy demand is growing rapidly as the developing countries are becoming industrialized. The potential for further growth is extremely large. A world population of 10 billion with a per capita energy consumption equal to that of the U.S. today would consume ten times more energy than the world consumes today. Even though most energy forecasts assume far less growth over the next fifty years, higher growth resulting in additional improvements in living standards and a consequent increase in political stability would be highly desirable. These lower estimates actually assume that economic growth in the first half of the 21st century will be smaller than that in the second half of the 20th century. Even so, growth in energy demand will still be very large. Even with the extensive use of alternative forms of energy, the demand for fossil fuels will increase significantly.
Unless environmental considerations artificially limit the use of fossil energy, there is no end in sight for the demand for fossil fuels. Combustion of such quantities of fossil fuels could drive atmospheric carbon dioxide levels very much higher. The available 10,000 Gt of carbon correspond to 4700 ppm of atmospheric carbon dioxide. While the detailed effects of carbon dioxide on climate and environment are still debated, it is known that carbon dioxide is a greenhouse gas which can cause climate change. Carbon dioxide affects the acidity of the ocean, it is of physiological importance and thus can directly affect the ecological balance of species. To continue current energy consumption patterns could eventually lead to a doubling of natural carbon dioxide levels. To stabilize carbon dioxide at 600 ppm would require a drastic reduction in carbon dioxide emissions, ultimately to about 30 percent of those of 1990. For 10 billion people sharing in such a carbon dioxide budget, the per capita allowance would come to about 3 percent of that of the average U.S. citizen today.
In summary, it appears to be extremely difficult to stop the growth of fossil energy demand, yet to stabilize atmospheric carbon dioxide levels would require a drastic reduction in carbon dioxide emissions. The logical solution appears to be methods of collecting and subsequently disposing of the gas after it has been generated. While it is acknowledged that it is easier to collect carbon dioxide from a concentrated stream than from a dilute stream, it has actually been suggested that carbon dioxide could be collected from the atmosphere to accomplish these objectives. See Lackner et al., "Carbon Dioxide Extraction From Air: Is it an Option?", Proceedings of 24th International Technical Conference on Coal Utilization and Fuel Systems, March 1999, Clearwater, Fla.
Given the expected increases of carbon dioxide in the atmosphere, it is clearly desirable to separate this gas from emissions by power plants or other sources, or even from the atmosphere itself, in order to dispose of or sequester carbon dioxide. Sequestration of carbon dioxide means its removal or segration from the atmosphere for a significant period of time, if not permanently. There are various approaches, including disposal in the deep ocean, injection into underground reservoirs and chemical stabilization as carbonate minerals. It is becoming increasingly important to prevent emissions from systems involving the combustion of fossil fuels from increasing the proportion of carbon dioxide in the air. Such removal and disposal, whether viewed as permanent sequestration or longterm term segregation, has economic value which can be awarded by national authorities as tax or pollution credits. For example, Norway presently levies a tax of over $50 U.S. per ton on carbon dioxide emissions. (See "Technology to Cool Down Global Warming," infra.) Equivalent amounts may be awarded to organizations sequestering carbon dioxide from combustion processes.
A significant fraction of the crude oil fed to a refinery consists of heavy material generally having a high content of sulfur. This material is oftentimes an environmental liability to the refinery with high disposal costs. Recently it has been considered that a more economical solution to the problem is to convert the heavy crude oil to synthesis gas using partial oxidation (POX).
The partial oxidation (POX) reaction can be expressed as: EQU CH.sub.z +0.5O.sub.2.fwdarw.z/2H.sub.2 +CO
where z is the H:C ratio of the hydrocarbon feedstock. The water gas shift (WGS) reaction also takes place: EQU H.sub.2 O+CO.rarw..fwdarw.H.sub.2 +CO.sub.2
The synthesis gas can then be used as fuel in a gas turbine to generate electrical power. An example of this approach is the api Energia S.p.A integrated combined cycle plant (IGCC) described in the Dec. 9, 1996 issue of the Oil & Gas Journal. In many instances, it is not desirable or practical to use all of the synthesis gas produced in the POX reactor for production of electricity. In these instances it may be desirable to convert some or all of the synthesis gas to liquid hydrocarbons which are free of aromatics and sulfur using Fischer-Tropsch (FT) chemistry.
The Fischer-Tropsch (FT) synthesis reaction is expressed by the following stoichiometric relation: EQU 2n H.sub.2 +nCO.fwdarw.C.sub.n H.sub.2n +nH.sub.2 O
The aliphatic hydrocarbons produced by the Fischer-Tropsch reaction have an H:C atom ratio of 2.0 or greater.
Fischer-Tropsch catalysts such as iron-based composites also catalyze the water gas shift (WGS) reaction: EQU H.sub.2 O+CO.rarw..fwdarw.H.sub.2 +CO.sub.2
If all of the water produced in the FT reaction were reacted with CO in the WGS reaction, then the overall consumption of hydrogen would be one-half of the consumption of carbon monoxide. If none of the water were reacted in the WGS reaction (no WGS activity) then the consumption of hydrogen would be twice the consumption of carbon monoxide.
The oil produced in the FT reaction can be blended and processed with the lighter refinery crude oil, thereby lowering the average aromatic and sulfur content of distillate fuels.
Due to the relatively low hydrogen content of the heavy crude oil, any FT catalyst useful in converting synthesis gas produced by partial oxidation of heavy crude oil must possess some water gas shift activity. Therefore, modern cobalt-based FT catalysts which have very little WGS activity cannot generally be used when the POX feedstock is a heavy crude oil, coke or coal. However, iron-based catalysts as described in U.S. Pat. No. 5,504,118 have high WGS activity and are preferred for use with low-hydrogen feedstocks.
For a natural gas feedstock which has a high H:C ratio, U.S. Pat. Nos. 5,620,670 and 5,621,155 teach that carbon dioxide recycle back to the synthesis gas producing step (either POX or steam reforming) decreases the excessively high H.sub.2 :CO ratio of the synthesis gas and increases the yield of Fischer-Tropsch (FT) hydrocarbons and the attendant carbon conversion efficiency.
In the case of low H:C ratio feeds, steam reforming is not a viable means for producing synthesis gas due to the inevitable formation of carbon when using these high carbon feedstocks. Carbon deposition on a reforming catalyst cannot be tolerated. Also, solid fuels are unsuitable for steam reforming. Thus, the only viable option for gasifying high C:H feeds is POX.
In the instant case, the aforementioned carbon dioxide recycle back to a POX reactor is not useful due to the lack of sufficient hydrogen.
Another means for increasing the hydrocarbon yield and carbon conversion efficiency of a system is to recycle part of the tail gas to the inlet of the POX unit. However, the amount of tail gas recycle is limited by the resulting low H.sub.2 :CO ratio in the synthesis gas produced in the POX caused by the large amount of CO.sub.2 in the tail gas.
The use of combined partial oxidation and Fischer-Tropsch reactors permits the conversion of a variety of high-carbon solid and liquid fuels to liquid hydrocarbons and other products which have lower C:H atom ratios and can thus be combusted or otherwise used with net lower emissions of carbon dioxide to the atmosphere. In the present invention, carbon dioxide can be efficiently removed from tail gases in the process and sequestered to reduce the net carbon dioxide emissions. Due to environmental and political considerations, there is increasing interest in reducing carbon dioxide emissions associated with combustion energy, and in trapping and sequestering such gases as are emitted. See "Technology to Cool Down Global Warming," Chemical Engineering, January 1999 (pp. 37-41). Because of these inherent advantages, Fischer-Tropsch technology is attracting increasing attention as a means for utilizing resources such as coal in efficient and environmentally friendly ways. Countries such as China and India, having large coal reserves and needs for liquid hydrocarbon fuels, could benefit immensely from such processes. See, e.g. Arthur W. Tower III, "Fischer-Tropsch Technology," published by Howard, Weil, Labouisse, Friedrichs of New Orleans, La., Dec. 18, 1998. See also "State of the Art in GTL Technology," presented by Dr. Joe Verghese of ABB Lummus Global at the Gas to Liquids World Forum, London, November 1998.
Information about Orimulsion.RTM., an aqueous emulsion of bitumen produced in Venezuela, can be found in various publications, including A. R. Jones' "The Commercial Combustion of Orimulsion," in the book Combustion & Emissions Control III, ed. M. Adams, Institute of Energy, London 1997 (pp. 318-339). See also Franzo Marruffo et al., "Orimulsion an alternative source of energy," presented at the 22nd International Technology Conference Coal Utility Fuel Systems at Clearwater, Fla., March 1997 (Coal & Slurry Technology Ass'n), pp. 13-24. Also pertinent is Rivalta et al., "Orimulsion.TM.--A New Fuel for Power Generation and Future Feedstock Use," Polymer News, Vol. 21, No. 10 (pp. 342-344).
U.S. Pat. No. 4,549,396 (Mobil Oil) discloses a process of converting coal to synthesis gas by partial oxidation with air, then converting the synthesis gas to liquid and gaseous hydrocarbons. The gas and liquid products are both used in a gas turbine to generate electrical power.
U.S. Pat. No. 4,433,065 (Shell Oil) discloses a process for converting pulverized coal to a synthesis gas, which is catalytically converted to a gas containing hydrocarbons. Part of the product gases are recycled to the gasification stage.
U.S. Pat. No. 4,092,825 (Chevron Research) discloses a process of gasifying solid carbonaceous feedstocks to form a synthesis gas, a portion of which is contacted with a Fischer-Tropsch catalyst to form condensable hydrocarbons. A second portion of the synthesis gas can be combusted to generate electrical power, while the condensable hydrocarbons are used as fuel to generate more power to meet peak loads. This patent is a C.I.P. of U.S. Pat. No. 3,986,349.
U.S. Pat. No. 3,972,958 (Mobil Oil) discloses an integrated process for converting coal to high octane gasoline by gasifying the coal to form a synthesis gas containing methane, then contacting the gas with at least one catalyst to form products including gasoline and light hydrocarbons.
Gray and Tomlinson of Mitretek Systems disclose in "CO.sub.2 Emissions from Fischer-Tropsch Fuels," presented at Fuels, Lubricants, Engines and Emissions meeting (sponsored by EFI and DOE) at Tucson, Ariz. on Jan. 18-20, 1999 a "coproduction cofeed" concept. Coal-derived synthesis gas is reacted in a liquid synthesis reactor to form liquid hydrocarbons, and unreacted synthesis gas is combined with natural (gas for combustion in a downstream combined cycle power generation unit.
U.S. Pat. No. 5,324,335 (Applicants) discloses the use of Fischer-Tropsch liquids as a diesel fuel additive.
U.S. Pat. No. 5,500,449 (Applicants) discloses a method of recovering a heavy Fischer-Tropsch wax and thermally cracking the wax to produce diesel and naphtha fractions.
U.S. Pat. No. 5,504,118 (Applicants) discloses methods for manufacturing and activating iron-based Fischer-Tropsch catalysts.
U.S. Pat. No. 5,506,272 (Applicants) discloses Fischer-Tropsch diesel fuel products.
U.S. Pat. No. 5,543,437 (Applicants) discloses methods for producing Fischer-Tropsch products from coal-derived synthesis gas. The products are produced at varying rates due to varying amounts of the synthesis gas being fed to a power generation facility.
U.S. Pat. No. 5,620,670 (Applicants) discloses a process of producing synthesis gas in a steam reformer, reacting the synthesis gas in a Fischer-Tropsch reactor, then separating carbon dioxide and recycling same to the reformer to enhance carbon conversion efficiency and product yield.
U.S. Pat. No. 5,621,155 (Applicants) discloses methods of producing synthesis gas which is reacted in a Fischer-Tropsch reactor, then separating and recycling carbon dioxide to the steam reformer or partial oxidation reactor or recycling light hydrocarbons from the Fischer-Tropsch reactor to the reactor inlet, all to increase carbon conversion efficiency.
U.S. Pat. No. 5,645,613 (Applicants) discloses the use of Fischer-Tropsch liquids as a blending stock for diesel fuel to produce oxygenated diesel fuels.
Even though the technology for conversion of high-carbon feedstocks to synthesis gas and the subsequent production of Fischer-Tropsch liquids is well developed, the growing demand for energy coupled with the need to limit emissions of "greenhouse gases" and/or to sequester carbon dioxide which is emitted by combustion processes create a need for more efficient and flexible processes which can meet the demand for power and hydrocarbon production while separating carbon dioxide for sequestration or disposal.